The subject matter disclosed herein relates to a magnetic resonance imaging (MRI) apparatus and a navigator data analyzing method, and particularly to a magnetic resonance imaging apparatus which executes scans for transmitting RF pulses to a subject within a static magnetic field space and acquiring magnetic resonance signals from the subject thereby to produce an image about the subject, and a navigator data analyzing method for analyzing navigator data obtained by executing scans, for detecting the position of each tissue in a subject.
A magnetic resonance imaging apparatus executes scans for applying an electromagnetic wave to a subject lying within a static magnetic field space thereby to excite spins of proton in the subject by a nuclear magnetic resonance phenomenon and acquiring magnetic resonance signals generated by the excited spins. This is an apparatus that generates a slice image with respect to a tomographic plane of the subject, based on the magnetic resonance signals obtained by the scans.
There is a case in which body-motion artifacts occur in the generated slice image where body motion occurs in the subject upon imaging the subject using the magnetic resonance imaging apparatus. When, for example, the heart or abdominal region of the subject is imaged, body motion artifacts occur due to body motion such as breathing exercises, cardiac motion or the like, thus degrading the quality of the image.
Thus, there have been proposed methods for solving the problem of the degradation in the image due to the body motion artifacts. One method thereof is that upon imaging under normal respiration, for example, an excitation section of a subject is corrected in real time according to a change in the position of the diaphragm and each magnetic resonance signal is always measured from the same section, thereby preventing the degradation in the image due to the body motion artifacts. An imaging sequence is changed or imaging data is selected through the use of acquired navigator echoes, thereby preventing degradation in image quality due to body motion artifacts (refer to, for example, Japanese Unexamined Patent Publication No. 2007-111188 and Japanese Unexamined Patent Publication No. 2007-98026).
As a method for detecting the position of the diaphragm, which is used in these techniques, there has been known a method for tracking the motion of the diaphragm using navigator echoes and performing respiratory synchronization and gating using acquired navigator data (refer to, for example, Japanese Unexamined Patent Publication No. Hei 10(1998)-277010).
As navigator data analyzing methods, there have been known, for example, an Ahn method, an LSQ method, an edge detecting method, etc. (refer to, for example, Yiping P. Du, Manojkumar Saranathan, and Thomas K. F. Foo. “An Accurate, Robust, and Computationally Efficient Navigator Algorithm for Measuring Diaphragm Positions”, JOURNAL OF CARDIOVASCULAR MAGNETIC RESONANCE Vol. 6, No. 2, pp. 483-490, 2004).
As a result that as shown in a coronal image of FIG. 18, an imaging area IA for executing an imaging scan to acquire imaging data has overlapped with a navigator area NA corresponding to the position of acquisition of navigator data, signal disturbance due to slice interference occurs in the acquired navigator data. As indicated by a broken-line area of FIG. 19, noise occurs in a signal intensity profile obtained by plotting the relationship between signal intensity I of acquired navigator data and a position L of a navigator area. Here, the broken-line area shown in FIG. 19 indicates a signal intensity profile corresponding to a portion where the imaging area IA and the navigator area NA shown in FIG. 18 overlap. In doing so, it became difficult to obtain a stable analytic result by the conventional navigator data analyzing method shown above.